This invention relates to the fabrication of ceramic articles, and, more particularly, to the preparation of articles having improved green strength.
Many techniques have been developed to fabricate ceramic materials into useful articles. The most commonly used is the particulate approach. This process involves compaction of a powder of the ceramic or suitable precursors, and densification at elevated temperature. The major shape forming processes based on the particulate approach are pressing (e.g., uniaxial, isostatic, hot isostatic), slip casting (e.g., vacuum casting, gel casting), tape casting (e.g., doctor blade), and plastic forming (e.g., extrusion, injection molding, rolling).
In many cases, an organic additive, termed a "binder", is used to assist in shape forming of the partially compressed but unfired ceramic article, termed a "green state" compact. The binder is mixed with the ceramic powder, and the mixture is compressed to create the green state compact that resembles the final article but is typically somewhat oversize to account for subsequent shrinkage. The green state compact is thereafter heated to remove the binder by vaporization, a process termed "burnout", and cause the ceramic particles to bond together during densification, a process termed "sintering".
Without the aid of a binder, it is almost impossible to form ceramic powders into suitable green states, because the ceramic powders do not cold bond together. The organic binders function as temporary bonding media for ceramic bodies to provide green strength for handling, inspection, and green machining. Also, the binder imparts other required properties to a ceramic batch such as die lubrication and plasticity.
A binder system used for ceramic fabrication is typically composed of two or more components: a binder, a plasticizer, and other processing aids such as a dispersant and a surfactant. A binder is generally a high molecular weight polymer whose function is to coat the ceramic particles to provide flexibility and integrity to the green state compacts. The binder determines the general range of final binder system properties. A plasticizer is a small- to medium-sized organic molecule that decreases cross-linking among binder molecules. The plasticizer is used to improve the plasticity or fluidity of the ceramic-binder mixture. The other processing aids are minor organic additives used mainly to improve ceramic powder processing. For example, dispersants in binder systems are polymers that aid in dispersion of ceramic particles. Surfactants are used to enhance the wetting characteristics between the binder and the ceramic particles.
The criteria for selection of binders include considerations such as (1) conferring sufficient flexibility and green strength upon the ceramic powder, (2) wetting the powder to aid dispersion, (3) leaving a minimal amount of residue after removal, and (4) having an adequate storage life. The criteria for selection of plasticizers include (1) not reacting but forming a softened mixture with the binder, and (2) having low volatility. Binder systems may be broadly classified as thermoplastic or thermosetting. Thermoplastic polymers can be softened by heating to moderate temperatures, whereas thermosetting materials are hardened by heating. Selection of the binder system and its composition is dependent on the type of forming process to be conducted and the shape of the ceramic article to be fabricated.
Among the various forming techniques mentioned earlier, tape casting and plastic forming processes have been used extensively for fabrication of multilayer ceramic structures and intricate shapes. Binder systems used in tape casting and plastic forming processes are generally selected to be thermoplastic. In tape casting, which has been developed to fabricate thin ceramic layers, the use of a thermoplastic binder allows layers to be bonded together by lamination. In plastic forming, the binder thermoplasticity allows a dough-like mixture of the ceramic powder and additives with required formability to be produced. In tape casting and plastic forming, fabrication of a ceramic structure often involves forming the structure in the green state followed by firing the green structure at elevated temperatures to form a sintered rigid body.
A major difficulty experienced in tape casting and plastic forming processes is removing the organic material during firing. Binder removal or burnout becomes increasingly important as the size and complexity of the fired structures increase. The binder burnout phase is critical in that gases must be given sufficient time to escape the structure without causing mechanical damage such as blistering or delamination. During the burnout phase, the compact must retain sufficient strength to prevent warping, distortion, or slumping, even though the binder is being removed.
Thus, one requirement of the binder system is that it should confer sufficient plasticity upon the compact to permit green state forming. However, once the ceramic structure is formed into the required shape, the binder should impart sufficient rigidity during the initial stage of binder burnout. In general, thermoplastic binders initially soften then vaporize when heated. During the softening stage of binder extraction, thermoplastic binders may not be stiff enough to support the ceramic structure as its size, weight, and complexity increase, resulting in slumping, collapsing, or deforming before sintering is complete.
There is therefore an ongoing need for an improved approach for achieving a sintered article with a desired structure. The present invention fulfills this need, and further provides related advantages.